1. Field of the Invention
The present invention relates to an axial-flow fan, and more particularly to an axial-flow fan mounted to the electric unit of a microwave oven and adapted to cool the magnetron and high-voltage transformer of the microwave oven.
2. Description of the Conventional Art
Typically, an axial-flow fan includes a hub coupled to the rotating shaft of a motor fixedly mounted to the mounting section of a fan guide, and blades arranged around the hub and integral with the hub. The blades rotate along with the hub, thereby causing a fluid to flow axially.
Such an axial-flow fan have diverse design parameters depending on the appliance to which the axial-flow fan is applied. Where such design parameters are improperly determined, noises of an increased level are generated during an operation of the appliance to which the axial-flow fan is applied.
In particular, the axial-flow fan involves noises resulting from a blade vortex interaction(VBI) of the blades occurring during a rotation of those blades as a downstream one of the blades, which viewed in a rotation of the blades, is struck against a vortex stream created by an upstream one of the blades.
In the axial-flow fan, noises may also be generated due to the so-called blade passing frequency(BPF) of a fluid, passing the blades, exhibited when the fluid is struck against a fixed construction such as a guide fixedly mounted around the blades.
The blade passing frequency is expressed by an integer multiple of the product of the number of the blades by the revolutions per minute of the blades. Such a blade passing frequency is generated due to the striking of a fluid flow against the fixed construction around the blades occurring during the rotation of those blades. This blade passing frequency serves as a major frequency increasing the level of noises generated at the axial-flow fan.
Referring to FIG. 1, a microwave oven installed with a conventional axial-flow fan is illustrated. As shown in FIG. 1, the microwave oven includes a casing 1 defined with a cooking chamber 2. A magnetron 3 and a high-voltage transformer 5, which serve to generate microwaves, are mounted to the outer wall surface of the cooking chamber 2 at a desired portion of the cooking chamber 2. A waveguide 4 is arranged between the cooking chamber 2 and the magnetron 3 in order to guide microwaves to the cooking chamber 2. An axial-flow fan assembly 10 is fixedly mounted to the inner wall surface of the casing 1 at a desired portion of the casing 1 in order to cool the magnetron 3 and the high-voltage transformer 5.
As shown in FIG. 2, the axial-flow fan assembly 10 includes a suction guide 11 fixedly mounted to the inner wall surface of the casing 1 and adapted to assist in stably sucking a fluid, a drive motor 12 arranged upstream from the suction guide 11 and adapted to generate a rotating force, and an axial-flow fan 13 coupled to a rotating shaft of the drive motor 12 to receive the rotating force and adapted to suck air and to discharge the sucked air toward the magnetron 2 and the high-voltage transformer 5.
The axial-flow fan 13 is a fan axially sucking and discharging a fluid. This axial-flow fan 13 includes a hub 13a coupled to the rotating shaft of the drive motor 12 to receive a rotating force from the drive motor 12, and a plurality of blades 13b arranged around the hub 13a and integral with the hub 13a and adapted to move a fluid while rotating.
A general operation of the microwave oven including the above mentioned conventional axial-flow fan will now be described.
When the magnetron 3 generates microwaves in response to the application of electric power from the high-voltage transformer 5 thereto, the generated microwaves are supplied to the cooking chamber 2 via the waveguide 4, so that food disposed in the cooking chamber 2 is heated and cooked. Simultaneously, electric power is applied to the drive motor 12 adapted to drive the axial-flow fan 13 coupled to the rotating shaft of the drive motor 12, so that the axial-flow fan 13 rotates. During the rotation, the axial-flow fan 13 sucks ambient air, and discharges the sucked air toward the magnetron 3 and the high-voltage transformer 5, thereby preventing the magnetron 3 and the high-voltage transformer 5 from being overheated.
Now, design parameters for determining the structural shape of the axial-flow fan 13 will be described in detail.
As shown in FIG. 3A, the conventional axial-flow fan 13 has an outer fan diameter Dfxe2x80x2 of 108 mm and a hub diameter Dhxe2x80x2 of 30 mm. The number of blades in the axial-flow fan 13 is four. Also, the axial-flow fan 13 has a sweep angle xcex1xe2x80x2 of 28xc2x0. The sweep angle is an angle defined between the line, which connects an intermediate point of the leading edge LExe2x80x2 of each blade with an intermediate point of the trailing edge TExe2x80x2 of the same blade between the outer peripheral surface of the hub and the tip of the blade, and a Y-axis perpendicular to a rotating axis of the blade, that is, a Z-axis.
The leading edge of each blade is positioned at a forward portion of the blade when viewed in the rotating direction of the fan, and the trailing edge of the blade is positioned at a rearward direction of the blade.
In FIG. 3B, xe2x80x9cxcex3xe2x80x2xe2x80x9d represents a rake angle, that is, an angle of each blade forwardly or rearwardly inclined with respect to the flow direction of a fluid passing through the axial-flow fan 13 when viewed from the side of the axial-flow fan 13, that is, along the X-axis. The flow direction of the fluid corresponds to xc2x1Z-axis. The axial-flow fan 13 has a rake angle xcex3xe2x80x2 of 0xc2x0.
In FIG. 3C, xe2x80x9cxcex2xe2x80x2xe2x80x9d represents a pitch angle of each blade in the axial-flow fan 13. The pitch angle is an angle defined between a phantom line, that is, a chord line Cxe2x80x2, extending between opposite blade tips when viewed from the side of the axial-flow fan 12, that is, along the X-direction, and the Y-axis perpendicular to the rotating axis of the blade, that is, the Z-axis. The axial-flow fan 13 has a pitch angle xcex2xe2x80x2 of 21xc2x0xc2x12xc2x0 at the outer tip of each blade and 30xc2x0xc2x12xc2x0 at the inner tip of each blade.
The position of a camber line CBxe2x80x2 connecting intermediate points between the upper and lower surfaces of each blade is expressed by a polynomial equation for the distance between the camber line CBxe2x80x2 and the chord line Cxe2x80x2. The position on the camber line CBxe2x80x2 spaced apart from the chord line Cxe2x80x2 by a maximum straight distance is referred to as a xe2x80x9cmaximum camber positionxe2x80x9d MCPxe2x80x2. The maximum straight distance of the maximum camber position MCPxe2x80x2 is referred to as a xe2x80x9cmaximum camberxe2x80x9d MCxe2x80x2.
The ratio of the maximum camber MCxe2x80x2 to the length of the chord line Cxe2x80x2 is referred to as a xe2x80x9cmaximum camber ratioxe2x80x9d MCRxe2x80x2. The conventional axial-flow fan 13 has a maximum camber ratio of 5.2% at the outer blade tip and 7.2% at the inner blade tip. The maximum camber position MCPxe2x80x2 is defined at a point spaced apart from both the leading edge LExe2x80x2 and the trailing edge TExe2x80x2 by a distance of 0.5xc2x10.1 when the distance between the leading and trailing edges LExe2x80x2 and TExe2x80x2 is defined to be 1.
In the above mentioned axial-flow fan applied to microwave ovens, air sucked by the axial-flow fan exhibits the above mentioned xe2x80x9cblade passing frequencyxe2x80x9d while passing by the suction guide arranged at the suction section of the axial-flow fan, thereby generating noises. The level of such noises is also increased due to a blade vortex interaction of the blades occurring during a rotation of those blades as a downstream one of the blades is struck against a vortex stream created by an upstream one of the blades.
Therefore, an object of the invention is to provide an axial-flow fan capable of reducing noises generated during a suction of air, in particular, noises resulting from a xe2x80x9cblade passing frequencyxe2x80x9d and a xe2x80x9cblade vortex interactionxe2x80x9d.
In accordance with the present invention, this object is accomplished by providing an axial-flow fan including a hub coupled to a rotating shaft and integral with the rotating shaft, and blades arranged around the outer peripheral surface of the hub and integral with the hub, wherein the number of blades, a pitch angle, a rake angle, a sweep angle, a maximum camber, and a maximum camber position, which are included in design parameters for determining structures of the hub and blades, are appropriately determined. In particular, the sweep angle is determined using specific mathematical equations.
In accordance with an embodiment of the present invention, the axial-flow fan comprises, as design parameters thereof: an outer fan diameter of 110 mmxc2x110 mm; an inner/outer diameter ratio of 0.30 to 0.38, the inner/outer diameter ratio corresponding to a ratio of an outer hub diameter to the outer fan diameter; a number of the blades corresponding to five or more; a maximum camber ratio ranging from a value of 10.2 to 11.2% at an outer tip of each of the blades to a value of 4.2 to 5.2% at an inner tip of each of the blades; a maximum camber position defined at a point spaced apart from leading and trailing edges of each of the blades by a distance of 0.5xc2x10.1 when the distance between the leading and trailing edges is defined to be 1; a pitch angle ranging from an angle of 30xc2x0xc2x12xc2x0 at the outer blade tip to an angle of 43xc2x0xc2x12xc2x0 at the inner blade tip; a rake angle ranging from 5.5xc2x0 to 5.9xc2x0; and a sweep angle varying in a radial direction of the fan.